It has been shown that tumors have developed numerous ways to escape tumor specific immune responses. These mechanisms will ultimately not only enhance tumor growth, but also impair the effect of immune based therapies in cancer. Myeloid derived suppressor cells represent a recently identified cell population, which has been shown to impair tumor specific immune responses both in mice and human. MDSC can be divided in two subtypes (namely monocytic and granulocytic MDSC. We have been able to examine and define the regulation and function of murine MDSC by IFN-gamma and studied MDSC in different liver tumor models, models of liver inflammation and in response to anti-CD40 treatment.